Well I am curious as well. So I looked up the definition in Webster''s dictionary.

A light and elegantly furnished vessel, used either for private parties of pleasure, or as a vessel of state to convey distinguished persons from one place to another; a seagoing vessel used only for pleasure trips, racing, etc.

So I guess if you race and you can call ypour boat seagoing it is a yacht. Now all you have to define is what is seagoing ...